Abstract As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have enabled the formation of a protective oxide layer and thus reduced the extent of internal oxidation. Due to the extensive oxidation ingress along the large Laves phase, it is concluded that T14 is not suitable for applications in SHS at 800 °C.

Abstract This paper investigates particle erosion properties of wear protection layers at high temperatures. The result of the test, including the running conditions of an actual plant, is discussed using a chromium carbide-20wt%-NiCr (HVOF) cermet coating, Fe-Cr-Si-B (arc spray), Inconel (TIG), and Stellite No.6 (TIG). The results are summarized as: the highest beam wear resistance is shown by the chromium carbide-20%wt-NiCr (HVOF) cermet coating; no correlation can be established between erosion resistance and layer hardness; particle erosion properties of wear depend on elastic-plastic phenomena between the particle and the surface; the impact of solid particles does not cause any abrasion when the particles move onto the elastically rubbing area of the layer surface; and the solid particle impact seldom causes abrasion when the particles move to the plastic ploughing area. Paper includes a German-language abstract.

Abstract There has been a growing interest in the use of high velocity oxy-fuel (HVOF) thermal spraying to deposit protective coatings onto the surfaces of engineering components to allow them to function under extreme conditions. To date a number of studies have examined the influence of HVOGF process parameters on the microstructure and corrosion resistance of sprayed metallic coatings. However, much less is known about microstructure and corrosion resistance of HVOLF sprayed metallic coatings and particularly their behaviour in comparison with HVOGF deposits. Therefore the aim of this paper is to investigate the corrosion behaviour of two metallic alloys sprayed by both the HVOGF and HVOLF processes. Coating microstructures are described and the corrosion behaviour is compared with that of wrought Inconel 625, cast Stellite 6 and weld overlay Stellite 6. Paper includes a German-language abstract.

Abstract Fretting wear resistance of plasma spraying coating is not satisfied with operative demand. After remelting treatment, the plasma spraying CoCrW coating can take on excellent fretting wear resistance. This paper discusses the processes involved in remelting of plasma sprayed CoCrW layers using an electron beam. Structure and seizure resistance are compared with as-sprayed layers. The results show that oxides, porosity, and lamellar structures are eliminated and a metallurgical bond between layer and substrate could be achieved through this treatment. The remelted CoCrW layers showed in their composition cobalt in solution and carbides as reinforcing phases. The eating resistance could be increased by a factor of 13.3. Paper includes a German-language abstract.

Abstract This paper deals with experimental and theoretical aspects of the post-treatment of thermally sprayed layers with high-power lasers. Various layers, such as VPS-sprayed CoCrAlY layers, APS-sprayed zirconium dioxide, aluminum oxide, and hydroxyapatite layers, as well as APS-FeCr+TiC layers, are post-treated with a continuous carbon dioxide laser with a power range of several KW/centimeter square. The microstructure and the wear resistance of the layers before and after the laser treatment are examined. Another post-treatment technique used is the laser shock process, in which the laser has a pulse time of 10-30 ns and an output of 8-10 GW/centimeter square. This technique is used for HVOF-sprayed Al-SiC composites. The microstructure of the layers is characterized with an X-ray phase analysis, a light microscope, and a transmission electron microscope. The interaction between laser and layer is modeled with the aid of the "Fusion-2D" program. Paper includes a German-language abstract.

Abstract Thermally grown oxides (TGO) form at interfaces between the upper thermal insulation layers (TBCs) and the bond coats during operation. Macro cracks can be observed in many places in the TGO. The formation of macrocracks would reduce adhesion or lead to areas of stress peaks. To improve the adhesion, a modified powder for the boncoat was developed by adding cerium and silicon to the conventional MCrAlY. This paper investigates two kinds of TBC, which are a conventional CoNiCrAlY and a developed CoNiCrAlYCeSi bond coats. These TBCs are compared with regard to the oxidation behavior and the bond strength of the interface. To measure the adhesive strength, four-point bending tests are carried out with the normal TBC and the TBC with a modified bond coat. It is observed that the modified variant has a higher adhesive strength. The reason for this could be a reduction in the formation of mixed oxides. Paper includes a German-language abstract.

Abstract In this paper, investigations into the improved wear behavior are carried out on Plasma Transferred Arc-coated sleeves and bushings from electroplating. Basically, Stellite 6 and Tribaloy 800 with WC, WC-Co, TiC, NbC, and chromium carbide are analyzed using X-ray diffractometry, SEM, and micro-hardness measurements. Changes in the microstructure during the respective solidification processes are discussed. Using pin-and-disk tests in Zn-0.18Al baths at 470 deg C, conclusions are drawn about the wear behavior and correlated with microstructure analyzes and hardness measurements. Paper includes a German-language abstract.

Abstract Coatings have been prepared using the Diamond Jet hybrid and JP5000 high velocity oxyfuel (HVOF) systems with the objectives of improving corrosion resistance and reducing costs through increasing deposition efficiency. Models relating deposition efficiency, coating oxygen content and corrosion resistance to process parameters including fuel flow rate, oxygen flow rate and stand-off distance have been developed. A corrosion test cell has been designed and a procedure determined for studying the corrosion behaviour of large numbers of thermally sprayed coatings in an efficient manner. A significant improvement to the corrosion resistance of HVOF sprayed coatings has been achieved by spraying parameter optimisation and investigation of powder size and distribution. The project has also investigated the influence of spray angle on coating performance with a view to future onsite application. Coating materials tested and compared include nickel alloys Hastelloy C276 and 59, cobalt alloy Ultimet, duplex stainless steel S32750 and an experimental iron-based spray-fuse composition.

Abstract Commercially available coating techniques such as "open arc" and "spray & fuse" methods were used to compare the microstructural development with the plasma transferred arc (PTA) coating process for the hardfacing of NiCrBSi and Stellite 6 alloys. Denser eutectic structure was observed in the case of PTA coated layers of the Stellite 6 alloys than those of open arc weld-surfacing process. The shape of both carbides and borides in the 16C alloy coated by PTA processing were also obtained to have coarse morphology of carbides and borides while the "spray & fused" layers show a needle shape with finer distributions. Possible thermal history during each coating process is discussed. Based on microstructural observation, the hardness, wear resistance, and corrosion behaviors are reported. As expected, the alloy properties are directly related to their constituents of microstructure.

Abstract Improvement of the high velocity oxy-fuel deposition (HVOF) process in the last decade has led to coatings with significant improved microstructures for better protection against wear and corrosion. HVOF coatings of cermet and metallic materials provide protection against erosion and are therefore good alternatives to the use of high-priced material. This paper presents the results of a study undertaken within the core research program of the National Research Council of Canada technology group in surface engineering, "SURFTEC", in which the performance of ten HVOF erosion-resistant coatings were evaluated under both dry and slurry erosion. Ten different types of HVOF coatings were studied including: six grades of WC with either Co or a Ni based matrix, one grade of Cr3C2 in a Ni-Cr matrix, and three grades of metallic alloy: Ni alloy, Co alloy and a SS 316-L. Coatings performance was evaluated with respect to the volume ratio and composition of metallic binder in carbide coatings, type of carbide, coating microstructure, impinging angle and the size of the erodent particles. All coatings were produced using the HVOF IP5000 system controlled by the Hawcs-II controller. Slurry jet erosion tests were conducted using a 10 %w/w alumina particle/water slurry. Two alumina particle sizes, 320 and 80 grit (nominal grain diameters 35 μm and 200 μm, respectively) were used. The nominal impact velocity of the slurry was 15 m/s and the nozzle-specimen distance 100 mm. Dry erosion tests were conducted using 50 μm diameter alumina particles projected onto coated flat test coupons through a carbide nozzle of diameter 1.14mm with a particle velocity of 84 m/s at a feed rate of 2 ±1 g/min. let impingement angles of 90° and 20° were used for both dry and slurry erosion tests. The volume loss of material under various erosion conditions was related to the coating properties and microstructure. Results indicate that the coating behavior is dependent on the erodent particle size, the erosion impinging angle to some extent and for slurry erosion, to the corrosion resistance of the cermet matrix.

Abstract The tribological behavior of plasma-sprayed coating layers in Co-based alloy was investigated as a function of working distance, gun power, and gas flow. Surface roughness, microhardness, and wear volume were measured for different plasma spraying conditions, wear tests and hardness measurements were conducted at various temperatures, and the cross-section of wear tracks was observed using microscopy. The results obtained shed new light on the high-temperature wear mechanisms of plasma-sprayed coating layers in Co-base alloy.

Abstract Dynamic and thermal behaviors of particles injected into an HVOF supersonic flow were modeled using the Lagrangian formulation coupled with a moving grid system which allows to treat melting and solidification problems in a particle. The particle behaviors of alumina, Tribaloy-400 and MCrAlY were examined by numerical simulations. Velocities calculated by this model were compared with experimental measurements available in publications. For investigating the particle temperatures for which experimental data are hardly available, qualitative experiments were conducted with an MCrAlY powder. It was thus observed that unmelted particles predicted by the numerical simulation were also found in the microstructure of the sprayed MCrAlY coating. From this work, it was shown that the established model provides a reasonable prediction of dynamic and thermal particle behaviors in HVOF flows.

Abstract The results of an investigation into compound, structure and some properties of composite plasma coatings based on stellite with an addition of titanium nitride are presented. It is shown that the coating formed in a gas-air plasma is characterized by better wear resistance and resistance to scaling. The results of industrial test lend support to wide use of such coatings in machine parts and units that work under the simultaneous influence of impact, abrasive wear and high temperature.

Abstract This paper describes variations in the microstructure/composition and mechanical properties in plasma sprayed CoCrAlY coatings and a modified Rene 80 substrate of gas turbine blades operated for 21000 h under liquefied natural gas fuels. Substantial oxidation/carbonization occurred in near surface coatings of concave blades but not in convex coatings. Aluminum and nickel/titanium rich nitrides formed in concave coatings and substrates adjacent to the interface, respectively. Small punch (SP) specimens were prepared in order that the specimen surface would be located in the near surface and interface regions of the concave and convex coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave blade initiated at low strains up to 950 °C. The convex coatings had higher ductility than the concave coatings and substrate and showed a rapid increase in the ductility above 800 °C. Thus it is apparent that the oxidation/carbonization and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coatings is discussed in light of the operating temperature distribution and compared to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxidation.

Abstract Friction materials for braking devices on high-speed trains must be able to dissipate large amounts of energy levels and do so with a minimum amount of mass. Low thermal diffusivity materials such as coated steel discs are of particular interest to researchers in this field. This paper investigates the wear behavior of stellite coatings, based on cobalt or nickel alloys, applied to aluminum titanate pads. The coatings are assessed in terms of friction coefficient, contact temperature, and wear as compared to cermet coated steel discs.

Abstract Cavitation and erosion damage to hydroelectric turbines and pumps can be a major problem. The effectiveness of thermal sprayed cavitation-erosion resistant coatings for hydroelectric turbine and pumps was evaluated. The coatings evaluated were applied using High Velocity Oxyfuel (HVOF) and Plasma Spray systems. Hard facing cobalt based alloys were evaluated on coupons in the laboratory. Testing was performed using a cavitating jet erosion apparatus utilizing an operating pressure of 27.6 MPa. The results were compared to welded 308 stainless steel. Cavitation resistant austenitic stainless steel weld alloys were also evaluated. The results showed that the cavitation rate of the austenitic stainless steel weld alloys were as low as one third of the rate of the 308 stainless steel. The cavitation rate of the thermally sprayed hard facing coatings were more than three times higher than the rate of the 308 stainless steel. CERHAB glass enamel coatings containing 7 wt % wollastonite fibers were successful applied by using two combustion spray processes. The applied CERHAB coatings were successfully annealed using field portable heaters and may have application as a seal coat for thermal spray coatings applied in the field.

Abstract Protective coatings adapted for titanium alloys which come into frictional contact with one another are described. Among the many coatings investigated, the best ones are cobalt based. The coatings are sprayed only on one of the rubbing surfaces. During rubbing, a small part of the coating transfers to the unprotected titanium alloy surface. The rubbing pair is thus essentially composed of two cobalt alloy-based surfaces. This leads to low coefficient of friction and little or no damage to the rubbing surfaces. The coatings find particular application in the protection from adhesive and fretting wear, galling and seizure of gas turbine and jet engine parts or the like made from titanium alloys.

Abstract The plasma transferred arc process continues to be the coating method of choice for the application of cobalt base alloys onto valve and valve trim. Although new applications have been developed over the years, the process remains largely associated with the application of high performance, highly alloyed powders for relatively small parts or small areas of large parts. The use of the plasma transferred arc process for large volume application has been limited by the robustness and performance characteristics of the equipment and the use of cobalt. A new plasma transferred arc system (power source, torch and process controller) has been developed which allows the application of powder metal alloys at deposition rates of up to 40 pounds per hour. In addition, there has been a development of new non-cobalt powder alloys with excellent mixed corrosion and wear resistance properties. These capabilities have rendered the process technically and economically viable for large and demanding applications in the mining, power utility and steel industries. The new PTA system and the recent developments in powder alloys will be discussed. Reference will be made to specific applications in target industries.

Abstract Spherical CoNiCrAlY powders (~45 + 5 μm) were sprayed using a Jet Kote II HVOF gun working with propane and oxygen. The coatings sprayed on two base materials: Inconel 625 (Ni base) and MarM 509 (Co base) were between 1,6 and 3 mm thick. The aim of the study was to examine the adherence problems in relation with this thickness and the imposed thermal cycles (thermal diffusion treatment and aluminizing). The studied parameters were the expansion coefficients evolution before and after the different treatments and the residual stresses between coating and substrate measured by incremental hole dilling method.